When I use plastercraft, I have a bucket of water that the students rinse their hands in before they wash them at the sink. I tell them that no plaster can go down rhe sink because it will clog the sink and Clarence (our janitor) will not be a happy camper! The bowls of water that they use to dip the plaster in at their tables are collected at the end of class. I let them sit and the plaster settles at the bottom. I pore the water off into my bucket and scrap the plaster out of the bottom of the bowls into the garbage can. Eventually most all of the plaster goes into the garbage and the separated water goes down the drain.